This invention relates to the field of leadscrew driving. It has particular application to drivers for leadscrews which are installed in high resolution engraving systems. In such a system an engraving stylus may be mounted on a carriage which is carried by a leadscrew along a linear path in engraving contact with the surface of a rotating cylinder such as a gravure printing cylinder. The invention also has application to engraving systems wherein the cutting is performed by a high energy laser.
Leadscrews are provided with a long helical thread which passes through a threaded nut. The leadscrew is supported in a manner such that it is free to rotate but without any axial movement. The nut is fixed against rotation but is free to move axially along the leadscrew. A working head is secured to the nut, so that when the leadscrew rotates, the head moves in the axial direction. The screw may be driven by a motor assembly which responds to electronic pulses and which rotates through a defined angular increment for each applied pulse.
When used in a precision application of the type contemplated for this invention, a leadscrew must have an extremely accurate pitch. Small pitch errors in the leadscrew produce corresponding errors in the position of the working head relative to the workpiece. Sometimes the errors are cyclic in nature, and other times they are substantially local. The errors may occur abruptly or may accumulate gradually. In any event, they can cause production of an unacceptable workpiece.
Prior art engraving systems have used the most accurate (and expensive) leadscrews available and have simply tolerated errors which could not be machined out of the leadscrews. The use of shaft angle feedback does not solve the leadscrew pitch problem, because the work head location is not a linear function of the shaft angle being measured.
It is therefore seen that there is a need for leadscrew driving apparatus which is able to position a workpiece with improved accuracy. More particularly, there is a need for a leadscrew drive which has a linear relationship between a commanded net rotation angle of the leadscrew and the responding axial position of a work head.